1. Field of the Invention
The present invention relates to a mobile communication system, a UE (User Equipment), and a handover control method for their use and its program, and more particularly, to a handover control method for UEs of the CDMA (Code Division Multiple Access) system.
2. Description of the Prior Art
In a handover control of a UE of the CDMA system, when a UE 13 is to select cells 11 and 12 to which the handover is to take place as shown in FIG. 12, the reception level of a CPICH (Common Pilot Channel) to be received in common within the cells 11 and 12 is measured, and the cells 11 and 12 whose reception level is satisfactory are selected for the handover (see Japanese Patent Application Laid-Open No. 2002-27522 for instance).
More specifically, the selection is made on the basis of the CPICH RSCP (Received Signal Code Power) which is the code power of each of the cells 11 and 12 received by the UE 13, the proportion of the CPICH RSCP to the RSSI (Receive Signal Strength Indication) which is denoted as “CPICH Ec/No” and the CPICH path loss on the wireless path of each of the cells 11 and 12 among other factors.
On the other hand, actual communication (voice, video telephone, packet communication or the like) involving handovers use a channel different from CPICH, namely a DPCH (Dedicated Physical Channel) individually allocated each time one UE 13 performs communication.
Because of this difference between the CPICH used for selecting the cells 11 and 12 to which a handover is to take place and the DPCH used for actual communication, the cell 12, in which the CPICH is good in reception quality but the DPCH is poor in communication quality as viewed from the UE 13, may be selected for the handover.
In the conventional handover control described above, there is a limit to the number of cells to which a UE can perform a handover or to the number of receivers (fingers) among which rake synthesis is possible (e.g. six cells or 12 fingers), but much of the capacity within this limit may be occupied by cells whose DPCH quality is poor. It is desirable for a UE to be able to perform a handover only to cells each of which can provide a DPCH of high communication quality and to achieve synthesis by receiving more multi-path components from those cells.
Furthermore, transmission power on a DPCH from a cell whose DPCH is poor in communication quality not merely is unnecessary for this UE but also interferes with other UEs (e.g. other users), and therefore adversely affects the downlink capacity (user capacity) of the whole system.
What makes the matter worse is that transmission power on the DPCH of such a cell may be made excessively high by a power control request from a UE (for instance reaching the maximum transmission power of the base station) or, if there is a cell whose DPCH is good in quality, may be minimized by a power control request from a UE.